Re: [PATCH 0/7] Performance improvement for fanotify merge

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Feb 16, 2021 at 6:02 PM Jan Kara <jack@suse.cz> wrote:
>
> Hi Amir!
>
> Looking at the patches I've got one idea:
>
> Currently you have fsnotify_event like:
>
> struct fsnotify_event {
>         struct list_head list;
>         unsigned int key;
>         unsigned int next_bucket;
> };
>
> And 'list' is used for hashed queue list, next_bucket is used to simulate
> single queue out of all the individual lists. The option I'm considering
> is:
>
> struct fsnotify_event {
>         struct list_head list;
>         struct fsnotify_event *hash_next;
>         unsigned int key;
> };
>
> So 'list' would stay to be used for the single queue of events like it was
> before your patches. 'hash_next' would be used for list of events in the
> hash chain. The advantage of this scheme would be somewhat more obvious
> handling,

I can agree to that.

> also we can handle removal of permission events (they won't be
> hashed so there's no risk of breaking hash-chain in the middle, removal
> from global queue is easy as currently).

Ok. but I do not really see a value in hashing non-permission events
for high priority groups, so this is not a strong argument.

> The disadvantage is increase of
> event size by one pointer on 64-bit but I think we can live with that. What
> do you think?

Given the round size of fixes size events in v5.10, that would be a shame:

ls -l /sys/kernel/slab/*notify*event
lrwxrwxrwx 1 root root 0 Feb 17 12:23
/sys/kernel/slab/fanotify_fid_event -> :0000064
lrwxrwxrwx 1 root root 0 Feb 17 12:23
/sys/kernel/slab/fanotify_path_event -> :0000056
lrwxrwxrwx 1 root root 0 Feb 17 12:23
/sys/kernel/slab/fanotify_perm_event -> :0000064

Counter proposal:

struct fsnotify_event {
        struct list_head list;
        struct fsnotify_event *hash_next;
        unsigned int key;
        u32 mask;
};

It is quite strange that mask is a member of struct fanotify_event and
struct inotify_event_info to begin with.

Moving the mask member to struct fsnotify_event like that is not going
to change the resulting inotify/fanotify event size.

We can actually squeeze fanotify_event_type into 2 low bits of pid
pointer, and reduce the size of all fanotify events by one pointer,
because FANOTIFY_EVENT_TYPE_OVERFLOW is nice to have.
The overflow event can use FANOTIFY_EVENT_TYPE_PATH with a
NULL path values (as early versions of the patch did).

This is not worth doing with current round event size, IMO.

Thanks,
Amir.